"I know it’s scary when you first hear the word ‘pneumonia’ but thankfully I got to the doctor and hospital very quickly.”
The singer shared his “scary” diagnosis with fans after he was told by his doctor there was “no way in the world” he should continue on their upcoming UK tour dates.
Mark has missed their Newcastle gig and will continue to be out while he recovers.
"I know it’s scary when you first hear the word ‘pneumonia’ but thankfully I got to the doctor and hospital very quickly and got on the antibiotics as soon as possible, so everything is under control,” he told Instagram followers in an update.
"I have been informed that the quickest way to recovery is to rest and continue the antibiotics. I’m feeling a bit better every day.
"The nurses and doctors are taking amazing care of me so it should be all over in no time.
"This does however mean missing more gigs which is horrible but I’ll be back on stage as soon as I get the all clear.”
The star said his bandmates “will continue to fly the Westlife flag high for all of us.
"Please scream even louder than usual for those three legends in my absence!”
The Sligo star thanked followers for their well wishes and said he hopes to see them “back on the road very very soon.”
Mark has already updated fans to apologise for missing the Newcastle gig, revealing he “felt really feverish” following their time in Manchester.
"Please for me, have the time of your life!” he said.
Fans flocked to the comment section to wish the singer a speedy recovery and to send him love.
Some who were due to see the gang on stage in the UK said they were disappointed, but Mark’s health trumps it all.
"So devastated you will not be at Sheffield,” one said. “We will miss you but will do our best to sing our hearts out and support the boys in your absence.”
The quartet had arrived in Newcastle after a sell-out show at Manchester's AO Arena on Thursday evening but there was a blow to fans as Mark didn't join Shane Filan, Nicky Byrne and Kian Egan for Friday night's gig.
Westlife kept fans in the loop on Instagram.
They said: "Hi everyone. Unfortunately, under strict doctor's orders, Mark is still unable to get back on stage for tonight's show in Newcastle.
"The party will still go ahead and although we are all gutted he can't be there, he sends his love as always and hopes you all have the best night ever.
"Love, the boys x."